1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine which supplies a high voltage to a spark plug for each engine cylinder.
2. Description of the Related Art
As a conventional ignition apparatus for an internal combustion engine, there has been known one which includes a central core, a primary coil and a secondary coil both arranged outside of the central core, a magnet that is arranged in abutment with one end face of the central core and adapted to be magnetized in a direction opposite to the direction of a magnetic flux generated by energization of the primary coil, a side core that is arranged outside of the first and second coils and serves to cooperate with the central core and the magnet to form a closed magnetic circuit, a core cover attached to the side core, a case which receives therein the above-mentioned respective members, and a insulating resin filled into the case so as to fix the respective members to one another (see, for example, a first patent document: Japanese patent application laid-open No. H7-263256 (FIG. 1)).
In such a conventional ignition apparatus for an internal combustion engine, the magnet arranged to abut against the one end face of the central core is disposed at a location between a pair of parallel legs of the substantially U-shape side core. The magnet magnetically attracts the central core and the side core, so that a gap is generated between an end portion of the side core and the other end face of the central core at which the magnet is not arranged. In this case, there arises a problem that if this gap is large, most of the magnetic flux generated by energization of the primary coil leaks through the gap, resulting in reduction in magnetic efficiency.
Thus, there is a need to decrease the gap between the other end face of the central core and the end portion of the side core as much as possible, but it is necessary to press-fit the central core and the magnet into the side core depending upon dimensional variations of the central core, the side core and the magnet, as a result of which there arises another problem of poor assembling workability.